Pillowed flexible cube-corner sheeting and methods of manufacture

ABSTRACT

A flexible, durable, cube-cornered retroreflective article capable of accommodating the expansion and contraction of the underlying polymeric substrate where such article has a pillowed or curved microstructured member bonded in a regular pattern to a sealing member. The retroreflective article has normal and stressed states. In the normal state, the microstructured member is substantially parallel with the sealing member. In the stressed state, the microstructured member is either compressed or elongated and the sealing member is substantially flat. The retroreflective article may be attached to traffic control devices, such as polymeric barrels, cones, or tubes to direct and guide motorists through road construction areas.

TECHNICAL FIELD

This invention relates to a retroreflective sheeting, and moreparticularly a pillowed flexible cube-corner sheeting that accommodatesthe expansion and contraction of traffic control devices such aspolymeric barrels, cones, or tubes.

BACKGROUND

A retroreflective sheeting has the ability to redirect incident lighttowards its originating source. This ability has led to the widespreaduse of retroreflective sheetings on a variety of articles. Very oftenretroreflective sheetings are used on flat inflexible articles, such asroad signs and barricades. However, situations frequently arise whichrequire the sheetings to be used on irregular or flexible surfaces. Forexample, a retroreflective sheeting may be adhered to irregular surfacesof traffic control devices, such as polymeric barrels, cones, or tubes.These devices are used typically near roadway construction areas todirect and guide motorists. Also, a retroreflective sheeting may beadhered to a flexible substrate such as a road worker's safety vest. Insituations where the underlying substrate is irregular or flexible, theretroreflective sheeting desirably possesses good conformability andflexibility without sacrificing retroreflective performance.

There may also be situations where the underlying substrate expands andcontracts at a different rate than the retroreflective sheeting. Forexample, for a temperature decrease of 40° C. (92° F.), a trafficcontrol device such as a low density polyethylene barrel would contractby about 0.80%, based on a coefficient of linear thermal expansion of200×10⁻⁶ (m/mK) at 20° C. For the same temperature change, aretroreflective sheeting with a polycarbonate layer would contract byonly about 0.23%, based on a coefficient of linear thermal expansion of57×10⁻⁶ (m/mK) at 20° C. Thus, the barrel contracts almost 3.5 timesmore than the retroreflective sheeting. Because the retroreflectivesheeting is wrapped outside of the barrel, conventional sheetings maywrinkle and lift off the barrel in response to the temperature change.In these situations, it is desirable for the retroreflective sheeting toaccommodate the differences in thermal expansion and contraction withoutcompromising retroreflectivity and without lifting off the substrate.

There are basically two types of retroreflective sheeting: beaded andcube-corner sheeting. Beaded sheeting uses a multitude of glass orceramic microspheres to retroreflect incident light. Because themicrospheres are separate from each other, they do not restrict thesheeting's flexibility. However, a cube-corner sheeting typically uses amultitude of rigid, interconnected, cube-corner elements to retroreflectincident light, as shown in U.S. Pat. No. 5,450,235 (Smith et al.).Although different types of flexible sheeting are disclosed, noneaddresses the need to account for the differences in the coefficient oflinear thermal expansion between the substrate and the retroreflectivesheeting.

One way to produce a stretchable, flexible retroreflective sheeting isdisclosed in U.S. Pat. No. 3,992,080 (Rowland). That particular sheetingcomprises two flexible, stretchable strips of polymeric material. Thefirst strip is a transparent synthetic resin having a multiplicity ofminute cube-corner formations on one surface. The second backing stripis of a lesser length than the first strip when both are in a relaxedcondition. The backing strip is stretched a predetermined amount,typically 3% to 15%, before being bonded to the first strip ofcube-corners. After the bonding process, the backing material is allowedto relax thereby forming a puckered cube-corner sheeting. In the relaxedstate, the backing strip is in tension, while the cube-corner strip isin compression, to maintain the puckered appearance of the cube-cornerstrip. In applying this sheeting to a non-planar surface, such as abicycle handlebar, the retroreflective sheeting must be stretched to adegree sufficient to eliminate the puckering. However, excessivestretching results in distortion of the cube-corner formations and causea corresponding loss in retroreflectivity. This loss ofretroreflectivity translates into a loss in brightness, causing thesheeting to be less effective as a safety device.

In view of the disadvantages of conventional retroreflective sheetings,it would be desirable to provide a sheeting that accommodates thedifferent rates of expansion and contraction between the polymericsubstrate and the sheeting. It is also desirable to provide a sheetingthat is not susceptible to brightness loss because of overstretching, orbecause of other deformations in the cube-corner retroreflective layer.

SUMMARY OF THE INVENTION

The present invention provides a flexible retroreflective sheeting thatcompensates for dimensional changes in the substrate without sacrificingretroreflectivity. The present invention also eliminates any need torely on stretching the sheeting to a predetermined length to provideoptimal retroreflective performance.

In one embodiment, the present invention includes a microstructuredmember having a body portion and a multitude of cube-corner elementsattached to a first side of the body portion, a sealing member, anetwork of intersecting lines bonding the first side of the body portionand the sealing members together in a regular pattern of cells, andoptionally an adhesive. This retroreflective sheeting exhibits a normalstate where the microstructured member is curved and is substantiallyparallel to the sealing member, and a compressed state where themicrostructured member is arched and the sealing member is substantiallyflat.

In summary, the process of making the retroreflective sheeting includesproviding a microstructured member having a body portion and a multitudeof cube-corner elements projecting from the first side of the bodyportion, providing a sealing member, conveying these two members atapproximately the same speed with the cube-corner elements facing thesealing member, and bonding the first side of the body portion and thesealing member to each other in a regular pattern to form sealed cellseach having a curved microstructured member.

In accordance with this invention, the sheeting is useful foraccommodating expansions and contractions of traffic control deviceswithout compromising retroreflectivity and without causing wrinkles inthe sheeting. Because of its flexible nature, the sheeting is alsouseful for applications to polymeric surfaces, irregularly shapedsurfaces such as a bicycle helmet, and flexible surfaces such as asafety vest.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a cube-corner retroreflectivesheeting in a normal state in accordance with the present invention;

FIG. 2 is a cross-sectional view of a microstructured member;

FIG. 3 is a cross-sectional view of a cube-corner retroreflectivesheeting in a compressive state; and

FIG. 4 is a schematic representation of a bonding process for making thesheeting of the present invention.

These figures, which are idealized, are not to scale and are intendedmerely to be illustrative and non-limiting.

DETAILED DESCRIPTION OF THE INVENTION

The inventive retroreflective sheeting allows for expansion andcontraction of the traffic control devices without compromisingretroreflectivity, without wrinkling, and without delaminating. Thesheeting has a pillowed microstructured member. The term "pillowed" asused herein means the member has curvature. This pillowing allows themicrostructured member to arch to accommodate the contraction of atraffic control element that occurs when the ambient temperaturedecreases. Conversely, the pillowing allows the microstructured memberto flatten to accommodate the expansion of a traffic control device whenthe ambient temperature increases. Furthermore, the pillowedmicrostructured member allows for some elongation in the sheeting whichaids in its application onto traffic control devices. The overall effectof the invention is to provide a versatile retroreflective sheeting thatcan accommodate for the differences in expansion and contraction betweenthe device and a sheeting and that is easy to apply onto traffic controldevices.

The retroreflective sheeting of the present invention exhibits normaland stressed states. A "normal" state represents a condition whereneither the microstructured member nor the sealing member is in tensionor in compression. In such a normal state, the microstructured memberhas curvature and is substantially parallel with the sealing memberexcept where the two are bonded together. The term "parallel" meanslines, including curved lines, that are spaced an equal distance apart.The sheeting would typically be in a normal state after fabrication.

A "stressed" state represents a condition where the microstructuredmember has deviated from its normal state, and is in, for example, acompressed state or an elongated state. A "compressed" state occurs whenthe microstructured member has arched. An example of a compressed stateis where the sheeting contracts in response to a contraction of theunderlying substrate. The sheeting compensates for the contraction bycompressing and thereby arching the microstructured member. An"elongated" state occurs when the microstructured member has flattened,eliminating almost all curvature. An example of an elongated state iswhere the sheeting responds to an expansion of the underlying substrate.The sheeting compensates for the expansion by flattening themicrostructured member thereby expanding with the substrate. In bothstressed states, the sealing member is substantially flat.

I. General Structure of the Sheeting

FIG. 1 shows an example of a microstructured retroreflective sheeting 10of the present invention in a normal state. Retroreflective sheeting 10comprises a microstructured member 11 bonded to the first side 26 ofsealing member 24 through multiple bond lines 30. As seen in FIG. 1, themicrostructured and seling members have similar curvatures and are saidto be substantially parallel to each other. The bonding of the twomembers creates sealed cells 17 containing air. The cells form an arrayof polygons. Optional adhesive layer 32 with liner 33 may be laminatedto the second side 28 of the sealing member 24 to permit the sheeting tobe adhered to a surface when the liner has been removed.

FIG. 2 shows an example of microstructured member 11, taken at 2 in FIG.1, comprising a multitude of cube-corner elements 12 and a body portion14. The body portion 14 can include a land layer 16 and a body layer 18.The cube-corner elements 12 project from a first or rear side 20 of bodyportion 14. The cube-corner elements 12 and the body layer 18 compriselight transmissible polymeric materials. Light enters themicrostructured member 11 through the front surface 21. The light thenpasses through the body portion 14 and strikes the mutuallyperpendicular planar faces 22 of the cube-corner elements 12, andreturns in the direction from which it came as shown by arrow 23, as isknown in the art.

In a preferred construction, the cube-corner elements 12 and land layer16 are made from similar or the same kind of polymers, and the landlayer 16 is kept to a minimal thickness. The land layer 16, typically,has a thickness in the range of about 0 to 150 micrometers, andpreferably in the range of approximately about 1 to 100 micrometers.Body layer 18 typically has a thickness of approximately 20 to 1,000micrometers, and preferably in the range of about 50 to 250 micrometers.The cube-corner elements 12 typically have a height in the range ofabout 20 to 500 micrometers, and more typically in the range of about 60to 180 micrometers. Although the embodiment of the invention shown inFIG. 2 has a single body layer 18, it is within the scope of the presentinvention to provide more than one body layer 18 in the body portion 14.

FIG. 3 shows an example of a microstructured retroreflective sheeting10' in a stressed state while attached to a traffic control device 50.In particular, the sheeting is in a compressed state wherein themicrostructured member has arched in response to a compressive forceplaced upon it by contraction of the device 50. A temperature decreaseis one example of such a stress. When the ambient temperature decreases,the traffic control device contracts. A desirable sheeting must respondto this contraction without wrinkling, delaminating, or lifting off thesubstrate. It is believed that as a polymeric traffic control devicecontracts, the sealing member 24 relaxes slightly and themicrostructured member 11 arches upward to accommodate this contraction.

The elimination of wrinkles in the sheeting creates several advantages.For example, the retroreflectivity performance of the sheeting is notcompromised because there is no significant distortion of thecube-corner elements. Also, the adhesive on the sheeting and thepolymeric substrate remain in contact with each other. Because of thiscontact, dirt and water cannot come between the adhesive and thesubstrate, which may cause delamination of the sheeting. Themicrostructured and sealing members are discussed in more detail below.

II. Retroreflective Microstructured Member

The two main elements of the retroreflective microstructured member arethe cube-corner formations and the body layer. Both elements includepolymers which are light transmissive, meaning that they are able totransmit at least 70 percent of the light incident upon them.Preferably, these polymers transmit greater than 80 percent and morepreferably greater than 90 percent of the incident light. Furthermore,the microstructured member maintains good dimensional stability and highdegrees of retroreflectance under highly flexed conditions.

The cube-corner formations function as the retroreflective mechanism.There are many cube-corner configurations known in the art, see, forexmnple, U.S. Pat. Nos. 4,938,563 (Nelson et al.), 4,775,219(Appeldorn), 4,243,618 (Van Arnam), 4,202,600 (Burke et al.), 3,712,706(Stamm), and 4,588,258 (Hoopman). However in the practice of thisinvention, the cube-corner configuration described in Hoopman may bepreferred because it provides wide angle retroreflection along multipleviewing planes.

The polymeric materials that are used in the cube-corner elements tendto be hard and rigid and may be thermoplastic. Examples of thermoplasticpolymers that may be used in the cube-corner elements include acrylicpolymers such as poly(methyl methacrylate); polycarbonates; cellulosics;polyesters; polyetherketones; poly(etherimide); polyolefins;poly(styrene) and poly(styrene) copolymers; polysulfone; urethanesincluding aliphatic and aromatic polyurethanes; and mixtures of theabove polymers such as a poly(ester) and poly(carbonate) blend, and afluoropolymer and acrylic polymer blend.

Additional materials suitable for forming the cube-corner elements arereactive resin systems capable of being crosslinked by a free radicalpolymerization mechanism by exposure to actinic radiation. Such systemsare further described in, for example, U.S. Pat. No. 5,450,235 (Col. 6lines 34-68; Col. 7, lines 1-48).

The polymeric materials used to make the land layer may be the same asthe polymers that are used to make in the cube-corner elements, providedthat the land layer is kept to a minimal thickness. In most instances,the land layer is integral with the cube-corner elements. The polymersthat are used in the cube-corner elements and land layer can haverefractive indices which are different from the body layer. Although theland layer desirably is made of a polymer similar to that of the cubes,the land also may be made from a softer polymer such as those used inthe body layer.

The body layer protects the sheeting from the environment, and canprovide mechanical integrity to the sheeting. It also gives the sheetingthe ability to bend, to curl, or to flex. Preferred polymeric materialsused in the body layer are flexible and resistant to degradation byultraviolet (UV) light radiation so that the retroreflective sheetingcan be used for long-term outdoor applications. Examples of polymersthat may be used to make the body layer include fluorinated polymers;ionomeric ethylene copolymers; low density polyethylenes; plasticizedvinyl halide polymers; polyethylene copolymers; and aliphatic andaromatic polyurethanes. Commercially available polyurethanes include:PNO3-214 (from Morton International Inc.,. Seabrook, N.H.) or X-4107(from B. F. Goodrich Company, Cleveland, Ohio).

Combinations of the above polymers also may be used to make the bodylayer of the body portion. Preferred polymers for the body layerinclude: ethylene copolymers that contain units that contain carboxylgroups or esters of carboxylic acids such as poly(ethylene-co-acrylicacid), poly(ethylene-co-methacrylic acid),poly(ethylene-co-vinylacetate); ionomeric ethylene copolymers;plasticized poly(vinylchloride); and aliphatic urethanes. These polymersare preferred for one or more of the following reasons: suitablemechanical properties, good adhesion to the land layer, clarity, andenvironmental stability.

Colorants, UV absorbers, light stabilizers, free radical scavengers orantioxidants, processing aids such as antiblocking agents, releasingagents, lubricants, and other additives may be added to the body portionor cube-corner elements. These components are known in the art, and arefurther described in, for example, U.S. Pat. No. 5,450,235 (Col. 9,lines 46-68 and Col. 10, lines 1-14).

II. Sealing Member

In FIG. 1, the microstructured retroreflective sheeting 10 of thepresent invention also includes a sealing member 24. The sealing memberfunctions to provide a mechanism to form sealed cells 17 and to protectthe cube-corner elements 12.

Examples of polymers that may be used in the sealing member includepolyurethanes, polyethylene terephthalate, polyethylene copolymer,alkylene/alkyl acrylate copolymers such as ethylene/methyl acrylatecopolymer, ethylene/N-butyl acrylate copolymer, ethylene/ethyl acrylatecopolymer, ethylene/vinyl acetate copolymers, polymerically plasticizedpolyvinyl chloride (PVC), and polyurethane primed ethylene/acrylic acid(EAA) copolymer. The term "polyurethane" typically includes polymershaving urethane and/or urea linkages and such is the intended meaningherein. Also, polyurethane includes polyether polyurethanes, polyesterpolyurethanes, and polycarbonate polyurethanes. Blends of such materialsmay be used if desired.

An example of a suitable EAA material for use in the invention isPRIMACOR™ 3440 (from Dow Chemical Co., Midland, Mich.). It is acopolymer of ethylene and acrylic acid, the latter present at about 9weight percent of the total weight of ethylene and acrylic acid monomer;the copolymer has a melt index of about 10.

Polymerically plasticized PVC is considered a distinctly differentmaterial from monomericly plasticized PVC because plasticizers from theformer will not migrate from PVC. Polymerically plasticized PVC willremain flexible and will not cause deterioration in the opticalperformance of the retroreflective member.

Preferred polymers for the sealing member include polyetherpolyurethanes, polyester polyurethanes, polycarbonate polyurethanes, allof which may be aliphatic or aromatic. Also, blends of these may beused. A suitable blend comprises between approximately 50 and 99 weightpercent aliphatic polyester polyurethane with between approximately 1and 50 weight percent of a pigmented aromatic polyether polyurethane.One example of suitable blend comprises 60 weight percent of thealiphatic polyester polyurethane known as MORTHANE™ PNO3-214 (fromMorton International, Seabrook, N.H.) with 40 weight percent of apigmented aromatic polyether polyurethane. The pigmented aromaticpolyether polyurethane further comprises 50 weight percent aromaticpolyurethane known as ESTANE™ 58810 (from B. F. Goodrich Co., Cleveland,Ohio) and 50 weight percent titanium dioxide, previously compounded bysuitable means, such as in a twin screw extruder and subsequentlypelletized. Another example of a suitable polyurethane may be preparedby twin screw compounding between approximately 1 and 50 weight percentof titanium dioxide directly into an aliphatic polyurethane such asMORTHANE™ PNO3-214. These polymers are preferred for one or more of thefollowing reasons: suitable mechanical properties, environmentalstability, ease of processing, and good adhesion to the microstructuredmember. Suitable thicknesses for the sealing member is betweenapproximately 25 and 200 micrometers and preferably in the range betweenapproximately 50 and 130 micrometers.

III. Methods of Manufacturing

Retroreflective sheetings of the present invention can be made byproviding a microstructured retroreflective member having a body portionand a plurality of cube-corner elements projecting from a first side ofthe body portion; providing a sealing member; conveying themicrostructured member and the sealing member at approximately thespeed; and bonding the first side of the body portion and the sealingmember to each other in a regular pattern to form sealed cells eachhaving a curved microstructure member. The microstructured and sealingmembers may be bonded by thermal or ultrasonic means.

In a thermal bonding process, thermal energy and pressure are used tobond the microstructured and sealing members together. Typically, onemember is placed against a steel roll while the other member is placedagainst a rubber roll. The steel roll, referred to as the "embossingroll," has a raised ridge embossing pattern on its surface. Theembossing roll is usually heated to enhance the bond between the twomembers. The temperature at which the embossing roll is heated todepends on which member it contacts, and can range from betweenapproximately 193° C. and 260° C. (380° F. to 500° F.). Preferably, thetemperature range is from between approximately 204° C. and 243° C.(400° F. to 470° F.). The embossing roll and the rubber roll are allowedto come together with the rubber roll exerting pressure against theembossing roll. The pressures commonly used range from betweenapproximately 34 and 136 N/cm (20 to 80 lb_(f) /in). The raised ridgesbond the two members along a plurality of intersecting bond lines.

FIG. 4 shows a schematic representation of a thermal bonding processused to produce a preferred embodiment. Microstructured member 11 withits cube-corner elements 12 exposed is unwound from roll 34. Sealingmember 24 is unwound from roll 36. Typically, microstructured member 11is covered with a protective film 13 which is allowed to contact theembossing roll 38 turning at a surface velocity of V₁. Likewise, sealingmember 24 is covered with a protective film 25 which is allowed tocontact the rubber roll 42 turning at a surface velocity of V₂.Velocities V₁ and V₂ are approximately the same. Bonding between the twomembers occur at raised ridges 40 to form sealed cells. In these cells,there is typically air between the microstructured and sealing members.

The preferred thermal bonding process described in FIG. 4 is referred toas "front face" bonding, because the embossing roll contacts the front,or microstructured side of the article. As a result of front facebonding, the microstructured member of each cell has curvature and issaid to be pillowed. In contrast, if the placement of the members arereversed so that the sealing member contacts the heated embossing roll,the process is referred to as "back face" bonding.

Alternatively, ultrasonic energy may be used in place of thermal energy.However, the embossing roll would not be heated and the rubber rollwould be replaced with a suitable means for supplying ultrasonic energy,such as an ultrasonic horn and a power supply.

In either bonding processes, the microstructured member and sealingmember are bonded along bond lines that form a regular array ofpolygons. A suitable polygon allows the microstructured member to growin curvature or to flatten as a response to a dimensional change of thetraffic control devices. Preferred polygons include parallelograms, suchas rectangles or squares. The length of the rectangles can range betweenapproximately 5 and 150 mm (0.2 to 6 in); the width of the rectanglescan range from between approximately 5 and 25 mm (0.2 to 1 in). Thewidth is taken to be the longitudinal direction of the sheeting. In onepreferred embodiment, the retroreflective sheeting is bonded in arepeating rectangular pattern of 12.7 mm by 8.6 mm (0.50×0.34 in).

An adhesive may be laminated to one side of the sealing member. Thoseskilled in the art will recognize that care must be taken in selectingan adhesive that will adequately adhere to polymeric traffic controldevices because of their low surface energies. Suitable adhesive may bedistinguished by acceptably high shear strength, by acceptably high peeladhesion, and by resistance to delamination after an appropriate watersoak test. One suitable adhesive is a tackified synthetic rubber basedpressure sensitive adhesive.

After the retroreflective sheeting is fabricated, it can be applied topolymeric traffic control devices, such as barrels, cones or tubes. Awide variety of polymers may be used to fabricate traffic controldevices. Preferably, the traffic control devices will be selected from agroup of polymers that have a coefficient of linear thermal expansion inthe range of approximately 100×10⁻⁶ m/mK to 250×10⁻⁶ m/mK at 20° C.Furthermore, the ratio of the coefficient of linear thermal expansionbetween the traffic control device and the retroreflective sheeting isat least 1.5:1 and no greater than 6:1. Preferred polymers for trafficcontrol elements include low density polyethylene, high densitypolyethylene, polypropylene, plasticized polyvinyl chloride, and theircopolymers.

The sheeting may be applied to traffic control devices manually orthrough mechanical means as disclosed in U.S. Pat. No. 5,047,107 (Kelleret al.). In a manual application, tension is placed on the sheeting asit is being applied to the devices. An advantage of the presentinvention is that because of the pillowed microstructure member, thesheeting can exhibit some elongation, typically less than 3% withminimal cube-corner distortions. This elongation is enough to allow thesheeting to be guided in a straight line on a traffic control devicethereby further enhancing ease of manual application. Because theelongation comes from flattening the pillowed microstructured member,there is minimal cube-corner distortions and thus minimal reduction inbrightness.

IV. Examples

The following examples illustrate different embodiments of theinvention. However, the particular ingredients and amounts used as wellas other conditions and details are not to be construed in a manner thatwould limit the scope of this invention. All percentages are by weight,unless otherwise stated.

Example 1 (Comparative)

A retroreflective microstructured member was produced as follows. Moltenpolycarbonate resin (MARKLON™ 2407 from Mobay Corporation, Pittsburgh,Pa.) was cast onto a heated microstructured nickel tooling containingmicrocube prism recesses having a depth of about 86 micrometer (0.034in). These recesses were formed as matched pairs of cube-corner elementswith the optical axis canted or tilted 8.15 degrees away from theprimary groove, as generally described in U.S. Pat. No. 4,588,258(Hoopman). The nickel tooling thickness was 508 micrometers (0.02 in)and it was heated to 215.6° C. (420° F.). The molten polycarbonate, at atemperature of 287.8° C. (550° F.), was cast onto the nickel tool at apressure of about 1.03×10⁷ to 1.38×10⁸ Pascals (1500 to 2000 psi) forabout 0.7 seconds to replicate the microcube recesses. Simultaneouslywith filling the cube recesses, additional polycarbonate was depositedin a continuous land layer above the tooling with a thickness of about86 micrometers (0.0034 in). A previously-extruded 64 micrometer (0.025in) thick aliphatic polyester urethane (MORTHANE™ PNO3-214 from MortonInternational, Seabrook, N.H.) film was laminated onto the top surfaceof the continuous polycarbonate land when the surface temperature of theland was about 190° C. (375° F.). This aliphatic polyester urethane wasprotected by a 61 micrometers (0.024 in) thick polyester terephthalate(PET) film. The nickel tooling, along with the polycarbonate andlaminated polyurethane, was cooled with room temperature air for about18 seconds to a temperature between 71° C. and 88° C. (160° F. and 190°F.), allowing the laminate material to solidify to form themicrostructured member. This member, having a substantially flat firstside and a multitude of cube-corners on the second side, was thenremoved from the nickel tooling.

A sealing member was produced as follows. A blend of 60% aliphaticpolyester urethane (MORTHANE™ PNO3-214) and 40% aromatic polyesterpolyurethane (including 50% aromatic polyester urethane, ESTANE™ 58810from B. F. Goodrich Co., Cleveland, Ohio and 50% titanium dioxide,previously compounded in a twin screw extruder and pellitized) wasextruded. One side of the sealing member was protected by a 51micrometers (0.002 in) thick PET film.

Subsequently, the microstructured and sealing members were fed into anip at approximately the same speed between a steel embossing roll and arubber roll having a 75 Shore A durometer. The embossing pattern on thesteel roll was of rectangular configuration with dimensions of 0.86 cmby 2.54 cm (0.34×1 in).

The PET film of the microstructured member was allowed to contact therubber roll with the cube-cornered side exposed. The PET film of thesealing member was allowed to contact the steel embossing roll with thesealing member exposed (i.e. back face bonding). The steel embossingroll was heated to 216° C. (420° F.). The rolls turned at a speed of1.52 meters/min (5 feet/min) and the force on the nip was held at 43N/cm (25 lb/in). As the members passed through the nip, bonds werecreated between the exposed sealing member and the cube-corners of themicrostructured member. Both PET protective films were then removed. Apreviously coated 63 micrometer (0.0025 in) thick tackified syntheticrubber based pressure sensitive adhesive was laminated to the unbondedside of the sealing member.

The resultant retroreflective sheeting had a substantially smoothmicrostructured top surface. This sheeting was applied manually ontotraffic control device such as a barrel as generally described in U.S.Pat. No. 5,026,204 (Kulp et al.). The low density polyethylene barrels(from Traffix Devices Inc., San Clemente, Calif.) are about 4 feet talland had 5 tapered rings each slightly larger than the next and is moldedas one piece. The base of the barrel was molded separately.

The barrels were placed onto a mandrel rotating at 1.52 meters/min (0.5revolutions/min). They were heated to a surface temperature of 49° C.(120° F.). This heating simulated operating conditions used by somemanufacturers who apply retroreflective sheeting after flame-treatingthe barrels. Immediately after the heating, the sheetings were appliedmanually to the barrels.

As the barrels cooled to room temperature of about 21° C. (70° F.), thesheeting lifted off the barrel. It is believed that because the sheetinghad a smooth microstructured member, one way for the sheeting to respondto the barrel contracting was buckling and lifting off the substratethereby forming wrinkles.

Consequently, rain and dirt can accumulate behind these wrinkled areasand promote them causing a reduction in brightness. As the wrinkles getpromoted, there may be regions where the sheeting delaminates from thebarrels.

Example 2

The microstructured and sealing members were made in accordance withExample 1 except as described below.

The PET film of the microstructured member was allowed to contact thesteel emboss roll with the cube-cornered side exposed (front facebonding). The PET film of the sealing member was allowed to contact therubber roll with the sealing member exposed as shown in FIG. 4. Thesteel embossing roll was heated to 243° C. (470° F.). The rolls turnedat a speed of 1.52 meters/min (5 feet/min) and the force on the nip washeld at 86 N/cm (50 lb/in) to create bonds between the cube-corners andthe exposed sealing member. Both PET films were then removed. Apreviously coated 63 micrometer (0.0025 in) thick tackified syntheticrubber based pressure sensitive adhesive was laminated to the unbondedside of the sealing member.

The resultant retroreflective sheeting had a substantially pillowed orcurved microstructured member. The sheeting was laminated to a barrel asgenerally described in Example 2. As the barrels cooled to roomtemperature of about 21° C. (70° F.), the sheeting remained secured tothe barrel. It is believed that the pillows changed their shaped andarched to accommodate for the contraction of the barrel.

Measurements of the dimensions of the pillows were made to determine thegrowth in curvature. The two dimensions measured included the height ofthe pillows and the base of the pillows. The base is taken to be thelength one side of the rectangle. The height represented the distancefrom the midpoint of the base to the top of the microstructured member.

                  TABLE 1                                                         ______________________________________                                                   Height      Base                                                   Sheeting State                                                                           (cm)        (cm)   Ratio (H to B)                                  ______________________________________                                        normal     0.051       1.27   0.040                                           compressed 0.11        1.26   0.081                                           ______________________________________                                    

Referring to Table 1, the "normal" state referred to a sheeting of thisexample after adhesive lamination; the "compressed" state referred tothe sheeting applied on a barrel that had seen a temperature change from49° C. to 4° C. (120° F. to 40° F.). As Table 1 shows, the height tobase ratio approximately doubled as the sheeting changed from a normalto a compressed state in response to the contraction of the barrelresulting from the temperature decrease.

This invention may take on various modifications and alterations withoutdeparting from the spirit and scope thereof. Accordingly, it is to beunderstood that this invention is not to be limited to theabove-described but is to be controlled by the limitations set forth inthe following claims and any equivalents thereof.

I claim:
 1. A retroreflective sheeting comprising:(a) a microstructured retroreflective member having a body portion and a plurality of cube-corner elements projecting from a first side of said body portion; (b) a sealing member; and (c) a network of intersecting lines bonding said first side of said body portion and said sealing member to each other in a pattern of cells whereby said sheeting is changeable between:(i) a normal state wherein said microstructured member is curved and is substantially parallel to said sealing member; and (ii) a compressed state wherein said microstructured member is arched and said sealing member is substantially flat.
 2. The article of claim 1 wherein the retroreflective sheeting can elongate by up to 3% with minimal distortion of the cube-corner elements.
 3. The article of claim 1 wherein said body portion comprises a body layer.
 4. The article of claim 3 wherein said body layer comprises polymers that are flexible and ultraviolet light absorbing.
 5. The article of claim 1 wherein said cube-corner elements comprise polymers selected from the group consisting of acrylic, polycarbonate, polyester, polyurethane, and crosslinked acrylates.
 6. The article of claim 1 wherein said sealing member comprises polymers selected from the group consisting of polyurethane, polyethylene terephthalate, polyethylene copolymers, ethylene methyl acrylate copolymer, ethylene ethyl acrylate copolymer, ethylene vinyl acetate copolymer, and polyvinyl chloride containing a polymeric plasticizer.
 7. The retroreflective sheeting of claim 1, wherein said sheeting is adhered to a polymeric traffic control device.
 8. The article of claim 7, wherein said polymeric device comprises polymers having a coefficient of linear thermal expansion of between approximately 100×10⁻⁶ m/mK and 250×10⁻⁶ m/mK at 20° C.
 9. A retroreflective sheeting in a normal state comprising:(a) a substantially compressionless microstructured retroreflective member having a body portion and a plurality of cube-corner elements projecting from a first side of said body portion; (b) a substantially tensionless sealing member; and (c) a network of intersecting lines bonding said first side of said body portion and said sealing member to each other in a regular pattern of cells, wherein the retroreflective sheeting has an elongation of less than 3%.
 10. A retroreflective sheeting in a normal state comprising:(a) a substantially compressionless microstructured retroreflective member having a body portion and a plurality of cube-corner elements projecting from a first side of said body portion; (b) a substantially tensionless sealing member; and (c) a network of intersecting lines bonding said first side of said body portion and said sealing member to each other in a regular pattern of cells, wherein said body portion comprises a body layer.
 11. The article of claim 10 wherein said body layer comprises polymers that are flexible and ultraviolet light absorbing.
 12. A pillowed retroreflective article, made by the steps of:(a) providing a microstructured retroreflective member having a body portion and a plurality of cube-corner elements projecting from a first side of said body portion; (b) providing a sealing member; (c) conveying said microstructured member and said sealing member at substantially the same speed such that said cube-corner elements face said sealing member; and (d) bonding said first side of said body portion and said sealing member to each other in a regular pattern to form sealed cells each having a curved microstructured member, the pattern comprising an array of rectangular polygons having a length of between approximately 5 and 150 mm.
 13. The article of claim 12 wherein the rectangular polygons have a width of between approximately 5 and 25 mm.
 14. A pillowed retroreflective article, made by the steps of:(a) providing a microstructured retroreflective member having a body portion and a plurality of cube-corner elements projecting from a first side of said body portion; (b) providing a sealing member; (c) conveying said microstructured member and said sealing member at substantially the same speed such that said cube-corner elements face said sealing member; and (d) bonding said first side of said body portion and said sealing member to each other in a regular pattern to form sealed cells each having a curved microstructured member, wherein said article is adhered to a polymeric traffic control device that comprises polymers having a coefficient of linear thermal expansion of between approximately 100×10⁻⁶ m/mK and 250×10⁻⁶ m/mK at 20° C. 